GNU Astronomy Utilities

4.3.1 --usage

If you give this option, the program will not run. It will only print a
very concise message showing the options and arguments. Everything within
square brackets ([]) is optional. For example here are the first
and last two lines of Crop’s --usage is shown:

There are no explanations on the options, just their short and long
names shown separately. After the program name, the short format of
all the options that don’t require a value (on/off options) is
displayed. Those that do require a value then follow in separate
brackets, each displaying the format of the input they want, see
Options. Since all options are optional, they are shown in
square brackets, but arguments can also be optional. For example in
this example, a catalog name is optional and is only required in some
modes. This is a standard method of displaying optional arguments for
all GNU software.